This invention relates to a dual circuit vehicle hydraulic brake system and more particularly to a tandem master cylinder with a regulator for such a system.
German Patent DE-OS No. 2,604,664 discloses a tandem master cylinder with regulator for a vehicle hydraulic brake system comprising essentially a housing with a stepped bore and two pressure pistons movable therein. The pressure pistons form boundaries for two pressure chambers of which a first pressure chamber connects with the wheel brake cylinders of a front axle, and the second pressure chamber connects with the wheel brake cylinders of a rear axle with a brake force regulator being interposed therebetween. In this arrangement, the pressure pistons are dimensioned in such a manner that the pressure generated in the first pressure chamber associated with the front-axle brake circuit is higher than the pressure in the second pressure chamber associated with the rear-axle brake circuit. The lower brake pressure in the rear-axle circuit is reduced by the regulator when the pressure has reached a predetermined limit. The regulator includes essentially a stepped piston on which a control spring bears and a valve whose closure member is secured to an operating piston. The operating piston is subjected to the pressure of the front-axle brake circuit on its one end and to the non-regulated pressure of the rear-axle brake circuit on its other end. In the event of a failure of the front-axle brake circuit, the operating piston is operated to prevent closing of the regulator valve, i.e., there occurs no reduction of the brake pressure in the rear-axle brake circuit.
The arrangement of the above cited German Patent is costly from the point of view of machining and assembly because it necessitates a large number of individual components which sometimes are of complicated construction. This results in high manufacturing costs. Moreover, such a design is only realizable with master cylinders having a relatively large diameter.
As described in the foregoing, it is characteristic of the known arrangement that, as a result of the great difference in the effective surface of the two pressure pistons, the pressure generated in the second pressure chamber associated with the rear-axle brake circuit is substantially lower than the pressure in the first pressure chamber associated with the front-axle brake circuit. This is appropriate if both brake circuits are operable and the brake force at the vehicle rear wheels is to be lower than at the vehicle front wheels. This is, however, taken care of by the regulator provided when the pressure reaches a predetermined limit. However, in the event of a failure of the front-axle brake circuit because of a defect, the regulator is put out of circuit and the brakes of the vehicle rear wheels are subjected to the reduced brake pressure only. However, in the event of failure of the one brake circuit it is extremely important to achieve the best possible braking effect with the intact brake circuit.
The demand for a sufficient braking action of the rear-wheel brakes in the event of failure of the front-axle brake circuit also contrasts with the fact that the movement of the operating piston occurring in that event results in the consumption of a high amount of brake fluid, which is further increased by the displacement of the stepped piston. The brake pedal travel which is appreciable enough if a brake circuit has failed is thus further increased as a result of the additional fluid consumption.
The operating piston is only actuated in the event of a defect and, hence, the ring seals sealing the operating piston are not moved. It is known that seals which are not moved over a prolonged period of time are destroyed when suddenly loaded. It is then necessary to repair the master cylinder.